Along with the popularity of the Internet and the ever-faster telecommunications connections, the transmission of different types of files containing media has also increased. The use of multimedia messages employed in mobile systems has also increased since terminals equipped with a camera and multimedia characteristics have become more common. Due to fact that copying files is easy, illegal copying and editing of files also takes place. If the file does not contain copyright notice, it is difficult to protect the copyrights.
There are several methods for inserting copyright notice into a file. Examples of these are inserting a copyright marking into the file in such a manner that it is visible when the file is opened, a digital signature, and a digital watermark. In the first case, copyright notice can be inserted into an audio file, for instance, as metadata in such a manner that when the audio file is reproduced, the application used for reproduction also displays the copyright notice on the user interface of the application.
A digital signature is part of the Public Key Infrastructure that uses a third party to verify the identities of two communicating parties. When creating a digital signature, the signer calculates from the information to be signed a hash by using a one-way function, and encrypts it with his private key. The recipient also calculates a hash from the received information and compares it with the encrypted hash he has opened with the public key of the signer. If the hashes are identical, the information has been signed by the person whose public key was used to open the encrypted hash.
When using a digital watermark, copyright notice is inserted into the digital-format media file as extra hashed bits in such a manner that the copyright notice is only displayed with an application suitable for reading a digital watermark. In a conventional image processing application, the extra bits are displayed as noise.
However, the creator of the media file often does not insert copyright notice. For instance, copyright notice is not very often inserted into photographs taken by a private individual. Files containing media are often transmitted without copyright notice for instance by electronic mail or multimedia messages to several persons, and several copies may be produced of them. The copying and editing of files containing media may even lead to copyright infringements.